This invention relates to color data processing in recording color video signals to an image recording medium such as a video disc. More particularly, this invention relates to a method of color data mapping for recording color images onto a compact disc with extended graphic format (hereinafter abbreviated as CD-EG).
In Compact Disc specifications, a format of Compact Disc Graphic (hereinafter abbreviated as CD-G) is specified for displaying additional image information with digital sound. Since recording areas available for this purpose are limited, at most 16 colors selected from 4096 colors indicated with RGB.sub.444, as shown in FIG. 1A, are available. The RGB.sub.444 expresses colors compounded with three primary colors, i.e. R (red), G (green) and B (blue), respectively having a gradation expressed by 4 bits.
For recording a color image in a CD-G disc, these 16 colors for suitably expressing the color image are selected first from above-mentioned 4096 colors and recorded onto the disc as the information of a color look-up table (CLUT) in disc reproduction. In this case, the recording order of these 16 colors indicates the color number of them. Next, all pixels in the color image are converted into the respective color numbers and these numbers are recorded onto the disc a pixel data.
In disc reproduction, a CD-G player reads out these 16 colors and generates the CLUT, read out the pixel data, decodes the color number of each pixel and successively outputs a set of 3 primary color data by referring to the CLUT.
Recently, for more detailed image reproduction, the CD-EG format have been proposed. In this format, 256 colors selected from 262144 colors indicated with RGB.sub.666, as shown in FIG. 1B, are available. As the result, the pixel data appointing respective color numbers in the CLUT are expressed with 8 bits and these data are divided into two groups, one represents upper 4 bits, another represents lower 4 bits, and recorded onto the disc respectively.
Unfortunately, CD-G players only acknowledge top 16 color from 256 colors in the CLUT of the CD-EG and the lower 4 bits group from the pixel data of the CD-EG disc. Therefore, when these players reproduce the CD-EG disc of which the colors of the CLUT are freely selected, a poor color image may be reproduced. Therefore, it is desirable to select and to map above-mentioned 256 colors of the CD-EG in consideration for reproduction in CD-G systems.
As described above, on a recorded disc, each of the pixel data in the color image is expressed by one of the numbers corresponding with the colors in the CLUT.
That number is called a pixel value hereinafter. Furthermore, combinations of pixel values constituting a color image are called pixel data and the color look-up table (CLUT) called a color map hereinafter. For example, the color look-up table of the CD-G is expressed as the color map C.sub.16.
As stated above, at most 16 colors are selected from the RGB.sub.444 in the CD-G. Pixel values which express those 16 colors can be indicated in 4 bit-format because 16=2.sup.4. In the case of the CD-EG, at most 256 colors are selected from the RGB.sub.666. Pixel values which express those 256 colors can be indicated in 8 bit-format because 256=2.sup.8.
FIG. 2 Shows a relationship between colors and their gradations selected in the CD-G and the CD-EG. In the CD-G, there are 0 to 15 pixel values in decimal notation because at most 16 colors are used, the gradation of which are expressed in 4 bit-format. The area indicated by arrow marks FA and FB in FIG. 2 is thus interpreted in a color map for displaying a color image by the 16 colors at most.
On the contrary in the CD-EG, there are 0 to 255 pixel values in decimal notation because at most 256 colors are used, the gradation of which are expressed in 6 bit-format. The whole area in FIG. 2 is thus interpreted in a color map for displaying a color image by the 256 colors at most.
In the case of the CD-EG, two memory planes as a random access memory for holding pixel values are used in a CD-EG player which produces 256 colors. That is, as shown in FIG. 3A, pixel data of a color image in the CD-EG includes pixel values each expressed by 8 bits which include the lower 4 bits the format thereof being the same as that for the CD-G and the upper 4 bits. The pixel data is thus sent to a display apparatus with this format.
When a video signal based on such a format is reproduced by a CD-G player, those lower 4 bits are interpreted and then a color image is displayed. That is, for example pixel values "0" and "16" in decimal notation are expressed "00000000" and "00010000" in binary notation, respectively. The lower 4 bits are the same, so that the both are interpreted as a pixel value "0000" by the CD-G player.
Therefore, an image stored in the CD-EG can be vividly reproduced by the CD-G player, which produces at most 16 colors, only if colors of pixel values whose lower 4 bits are the same as each other are the same colors among the 256 colors in the CD-EG. For example, if colors whose pixel values "0", "16", "32", ..., in the color map of the CD-EG are the same colors, an image stored in the CD-EG can be vividly reproduced by the CD-G player. It is thus can be said that if colors are aligned by a cycle of a pixel value "16" in the CD-EG, an image stored in the CD-EG can be vividly reproduced by the CD-G player.
Gradation of a color is cut from 6 bits to 4 bits by the method mentioned above. This is shown in FIG. 3B where the upper 4 bits of 6 bits of color gradation in the CD-EG becomes color gradation in the CD-G.
The above explanation focuses on the pixel value. However, if the color gradation is focussed on, an image stored in the CD-EG can be vividly reproduced by the CD-G player if colors of pixel values where the upper 4 bits of the gradation is the same are the same color.
The following process is carried out in signal recording process to the CD-EG in order that color images recorded in the CD-EG might be reproduced by the CD-G player.
To express gradation of R, G, and B of dots constituting a color image to be recorded in 4 bit-format in binary notation to detect colors used for the image;
To select at most 16 colors suitable for displaying the image from the detected colors to compose a color map C.sub.16 by numbering those 16 colors with color numbers; and
To compose a pixel data table in which colors of the dots of the image are numbered the color numbers.
The following process is also carried out parallel to the above process.
To express gradation of R, G, and B of dots constituting a color image to be recorded in 6 bit-format in binary notation to detect colors used for the image;
To select at most 256 colors suitable for displaying the image from the detected colors;
To compose a histogram showing the frequency of use of the selected at most 256 colors;
To sort these selected colors according to the histogram and to number the selected colors with color numbers in order of the frequency of use to compose a color map C.sub.256 ; and
To compose a pixel data table in which colors of the dots of the image are numbered the color numbers.
Then, as shown in FIG. 4, a new color map Ccomp of 256 colors is composed by operating the color map C.sub.16 of 16 colors in the RGB.sub.444 for the CD-G and the color map C.sub.256 of 256 colors in the RGB.sub.666 for the CD-EG. The color map Ccomp is then recorded to the CD-EG with signals such as audio signals.
There are some conceivable methods for composing the color map Ccomp. One method is to map 256 colors of the color map C.sub.256 onto the color map Ccomp according to the gradation of each of the 256 colors being close to the gradation of a color among 16 colors of the color map C.sub.16 in color solid defined by three orthogonal co-ordinates (i.e. R, G and B axes). This method is called LEVEL I hereinafter. The 256 colors of the color map C.sub.256 are mapped onto the color map Ccomp in order of closeness in the color solid of the 256 colors to the 16 colors of the color map C.sub.16 in spite of that how many bits in binary notation of the gradation of the 256 colors are different from the gradation of the 16 colors expressed in 4 bit-format.
Another method for composing the color map Ccomp is to map colors among the 256 colors of the color map C.sub.256 onto the color map Ccomp according to the histogram. The colors to be mapped among the 256 colors are the colors whose upper 4 bits of 6 bits in gradation are equal to 4 bits in gradation of the 16 colors of the color map C.sub.16 and whose lower 2 bits of 6 bits in gradation are close to the 4 bits in gradation of the 16 colors in the color solid.
This method is called LEVEL III hereinafter. In LEVEL III, if there are colors more than 16 colors, among the 256 colors of the color map C.sub.256, whose upper 4 bits in gradation are the same as the 4 bits in gradation of a color of the color map C.sub.16 and whose lower 2 bits in gradation are different from each other, those colors are ignored.
The above-mentioned methods have disadvantages. First in LEVEL I, all the colors of the color map C.sub.256 are mapped onto the color map Ccomp, so that colors of the map C.sub.256 distant from the 16 colors of the map C.sub.16 in the color solid are also selected. This results in a poor color image if reproduced by the CD-G player.
Secondary in LEVEL III, the number of colors selected from the color map C.sub.256 to the color map Ccomp decreases very much because of ignored colors, so that this results in also a poor color image if reproduced by the CD-G player.
Furthermore in LEVEL III, the judgment is made as to whether the 4 bits in gradation of a color of the color map C.sub.16 is equal to the upper 4 bits of the 6 bits in gradation of a color of the color map C.sub.256. This results in a poor color image with lower average intensity corresponding to the number of colors of the color map C.sub.256 whose upper 4 bits are equal to the 4 bits of colors of the color map C.sub.16.